Ben Riley-Smith is a Telegraph political reporter covering the Scottish independence referendum. Get in touch via @benrileysmith or ben.riley-smith@telegraph.co.uk

Scotland decides: 17 days left – Team Salmond believe the polls are wrong

Far more people are planning to vote Yes than the polls show. That, in a nutshell, is what Alex Salmond's camp believe. And the thinking behind that belief is one of the most important hypotheticals underpinning this campaign.

Will enough new voters turn out for Yes to swing it their way?

Over the last 10 days, two of the First Minister's closest advisers have privately said they believe the polls are underplaying support for independence. Both had largely the same argument, which goes something like this.

People are only ever picked up by pollsters if they have a landline telephone or internet connection. All the signs are that turnout will be as high as 80 per cent come September 18 (way above the last Scottish election, which was around 50 per cent). Huge swathes of new voters – registering for the first time or coming back to politics after years of apathy – are not being registered by polls. And the vast majority of them will vote Yes.

It would be easy to dismiss straight off the suggestion that the pollsters are wrong. After all, people tend to downplay polls when they don't like the numbers. Plus recent electoral history reveals form in such complaints being disproved – for all the Republican rage over Nate Silver's increasingly positive predictions for Obama in the run up to the 2012 US presidential election, he called all 50 states right.

But given the weight Salmond's team puts on this issue, it deserves consideration.

Firstly, new voters. A few weeks ago, The Scottish Sun ran a spread revealing the way 90 "urban estates" planned to vote. Unlike the national polls, a majority of Scots on many estates – often traditional Labour strongholds – were planning to back independence.

More than 18,000 Scots were included in the snapshot, gathered by the left-wing Radical Independence Campaign (RIC). Some 44 per cent were for Yes vs 25 per cent for No (with 31 per cent undecided). Many of them, it's safe to assume, are now registered to vote thanks to RIC's drive, plus they will be on the Yes camp's list of doors to knock come voting day.

Secondly, lapsed voters. Less statistical evidence here, but the (supposed) rationale is this – people who haven't voted for decades are disillusioned with Westminster and the status quo. Given the chance for serious change, Yessers believe, they will turn out and vote for independence.

Pushed for a response recently, one senior pro-UK strategist could not have been blunter – people with mattresses in their gardens do not win elections. There was also a strong defence for how the No side's Get Out The Vote organisation has been whipped into shape since the turn of the year.

Alex Salmond will refocus the spotlight on this issue today, with the deadline on new voter registrations due to slam shut at midnight tomorrow.

"Hundreds of thousands of people, many of whom have never voted in their lives, are now engaged in the most exciting, participate and powerful debate in Scottish political history," he said in a pre-released statement.

Salmond also predicted over the weekend that the "missing million" – the SNP's tag for lapsed and unregistered voters – would speak as one come polling day in a way that will make "the Hampden roar seem like nothing".

Which is a neat rhetorical flourish. But it does make clear what is fast becoming one of the crucial imponderables of this campaign.

Who is right – the pollsters, who with three weeks left predict the majority of Scots will back the Union, or Team Salmond, who believe new voters missed by the polls will turn out in such numbers to sweep Yes to victory?